An article published in the journal “Nature” describes a research on the ultra-diffuse galaxy NGC 1052-DF2. A team of researchers led by Pieter van Dokkum of Yale University, used a number of telescopes to observe this galaxy’s composition concluding that it contains a very low amount of dark matter. The gravitational effects detected in the galaxies show that generally they contain an amount of dark matter much higher than that of ordinary matter but NGC 1052-DF2 is an exception and therefore must be carefully studied.
At the moment the cosmological model that gets the greatest consideration is called Lambda-CDM (Cold Dark Matter) paradigm, which among other things predicts that galaxies formed within dark matter halos. The observations made during the investigations on galaxies indicate that between the mass of the dark matter of that halo and the mass of the stars’ ordinary matter there’s a link given by a function that varies regularly with the mass.
For example, a galaxy similar to the Milky Way has a halo with an average mass 30 times higher than its stars’ mass. Not everything is still clear, on the contrary the question of dark matter is still very open and the discovery of a galaxy that seems to have a very limited amount of it can only be a further source of arguments.
The galaxy NGC 1052-DF2, not to be confused with its neighbor NGC 1052, is of the ultra-diffuse type, characterized by a very low density. Its size is similar to the Milky Way’s but contains an amount of stars estimated at 1/200th of the Milky Way’s. Several galaxies of this type have been cataloged and the observations suggest that they’re held together by dark matter but in the case of NGC 1052-DF2 there was a surprise.
The observations that led to the conclusion about the galaxy NGC 1052-DF2 began with the Dragonfly Telescope Array, a robotic instrument optimized for the detection of low-brightness sources built by Pieter van Dokkum and Roberto Abraham of the University of Toronto, another author of the article. That instrument revealed that NGC 1052-DF2 was an unusual galaxy.
At that point, the researchers conducted more observations of the galaxy NGC 1052-DF2 with the W.M. Keck Observatory to measure the velocity of the globular clusters inside it. It turned out that those clusters move slower than expected, a useful data to calculate the galaxy’s mass. The real surprise came when the amount of dark matter calculated was 1/400th of what was expected.
A further strangeness in the galaxy was found thanks to the Hubble Space Telescope, whose images show that NGC 1052-DF2 doesn’t have a noticeable central region nor spiral arms and a disk as spiral galaxies normally have. On the other hand, its globular clusters are twice the size of the typical ones seen in other galaxies.
It’s possible that the strange situation of the galaxy NGC 1052-DF2 was caused by a catastrophic event. Approximately 65 million light years from Earth and with an age estimated in 10 billion yeras, it’s part of a cluster dominated by its neighbor NGC 1052, which may have influenced the formation of NGC 1052-DF2. So far there are only speculations and this is the first galaxy in which the gravitational effects attributed to dark matter turned out to be very weak so the researchers are already studying other ultra-diffuse galaxies.
These anomalous cases can be useful to test cosmological models, including the alternative ones that don’t include the existence of dark matter. The Dragonfly Telescope Array is an ideal instrument to discover galaxies like NGC 1052-DF2 in a search that could add important information to better understand the issue.